This is a Phase I clinical trial of the ATP-sensitive potassium channel, PNU-83757 in treating erectile dysfunction. The primary objective of the study is to determine the highest tolerable dose of PNU-83757. The secondary objectives are 1) to determine the systemic pharmacokinetics parameters of PNU-83757; 2) to assess the linearity of the pharmacokinetics; 3) to obtain a preliminary estimate of the effective dose range; and 4) to collect data on the safety of the drug.